This invention relates generally to a screen printing machine having a stationary printing table and at least two gripper rails that are in motion at the same time. One rail is guided over the printing table in order to grip a new blank sheet at a feeding table during a printing operation, pull the sheet over the printing table after the printing operation, and position the sheet on the table. Subsequently, the rail is guided back to the feeding table.
Screen printing machines of the general type having a row of gripper rails on a joint conveyer are known in the art. The upper end of the conveyer extends in a plane above the printing table and the lower end extends in a plane below the printing table. The gripper rails, which are disposed transversely over the printing table with a gripped print sheet, are first guided over the printing table. The rails reach their starting position for gripping a new sheet only after angular deflection and return below the printing table. Screen printing machines of this type generally provide that these gripper rails also convey the printed sheet to a delivery station so that the deflection and return will start only after this conveyance. These machines are disadvantageous because the rail return path encompasses the whole lower part of the machine. Many gripper rails are required for one machine and the cost of construction and space requirements for machines of this type are relatively high.
Also known are other screen printing machines where a carriage is moved back and forth laterally of the printing table. The length of the carriage corresponds approximately to the length of the printing table and the carriage has gripper rails at each of its two ends that can operate on the same side. By means of this carriage, print sheets that are to be placed on the printing table can therefore be gripped by one rail and be conveyed over the printing table while the gripper rail disposed on the other side of the carriage can grasp an already printed sheet and convey it to the delivery point. However in this type of screen printing machine, it is a disadvantage that the carriage must move back empty for the purpose of gripping of a new sheet. Machines of this type therefore have relatively long idle time.
One objective of this invention is the provision of a screen printing machine that requires as few gripper rails as possible for the transport of print material on the printing table without the disadvantage of excessive idle times.
This and other objectives of the present invention are achieved by the provision of two gripper rails which are each separately guided over the printing table in a guideway but whose movements take place in opposite directions to one another. The guideways are designed in such a way that one of the gripper rails passes over the other one in the region of the center of the printing table. This design has the advantage that, during the transport of a sheet to be printed onto the printing table by a first gripper rail, the second gripper rail is already returning to its starting position. Therefore, during the removal of the printed sheet, a new sheet can be moved into position immediately.
It is advantageous for each gripper rail, to be connected with a driving means which is operated jointly with the driving means of the other gripper rail but in the opposite direction. It is also advantageous for the driving means to be connected to a joint endless conveyor to which the gripper rails are fastened at a distance corresponding to half the length of the endless conveyer.
This type of design ensures an opposing of movements without requiring separate drives for the two gripper rails. Advantageously, the endless conveyer is a rotating chain extending in the area of the guideways of the gripper rails. The gripper rails are provided at their two ends with guiding parts having pulleys running on guide rails serving as the guideways. This ensures exact guiding and steady movement of the gripper rails.
The opposing drive and the controlling of the lifting movement of one gripper rail over the other one may take place in different ways. It is contemplated that the driving and guiding wheels of the chain have horizontal axles and that the guide rails extend above one another in a vertical plane. This design has the advantage of a narrow construction. Tne guide rails in this case may advantageously be the inside edges of a slot extending laterally in an oblong chain case receiving the chain. The slot is designed symmetrically relative to the center of the longitudinal edges of the printing table and open in the direction of the printing table. This combines covering and protection of the chain to the outside with the arrangement of the guideways for the gripper rails. The slot must be wider in its center region than in its end regions to a degree that is sufficient for letting the guide parts of the gripper rails pass over one another.
In addition, biasing means are provided in order to press the guiding parts against the lower inside edge of the slot during movement in one direction and against the upper inside edge of the slot during the movement into the other direction. In this way, the passing of the guiding parts of the gripper rails and thus also of the gripper rails themselves in their path of motion is ensured. As biasing devices, guide tracks may be provided on which the pulleys of the guide parts are lifted while travelling in one direction, while during the return, they pass below this guideway. However, it is simpler and less susceptible to disturbances or misguiding when a pair of pivoted levers is provided as the biasing device. The levers are coupled to the chain at the joint pivotal point. The free ends of the pivoted levers are arranged in the guide part of the gripper rail so that they can be slid in parallel to the direction of movement of the gripper rails. The free ends of the pivoted levers are biased via a pressure spring or similar means toward a position such that they seek alignment with one another.
This design has the advantage that the pair of pivoted levers, which at the joint pivotal point is forcibly guided at the assigned end of the belt, constantly exercises a biasing force on the guide part which constantly attempts to bias the guide part in the direction of the assigned end of the chain. In this way, the contact of the guide part or its pulleys with the outside contour of the slot is always ensured. This design also has the advantage that at the two deflecting points of the chain, the pivoted levers are forced into their stretched position thereby relieving the pressure spring. The pivoted levers then swing into the opposite direction and again exercise a pulling force on the guide part as caused by the rotating motion of the chain. The guide part of each gripper rail arranged on the chain in this way is thus forcibly guided at the outer edge of the slot assigned to it. The guide parts therefore forcibly move past one another in the widened center area of the slot without touching. On both sides of this crossing point, the guide parts extend in such a way that the gripper rails, are guided flatly over the printing table in the desired manner.
It is advantageous to dispose the free ends of the pivoted levers at sliding blocks which are guided in a longitudinal guide in a guiding carriage serving as the guide part. The guiding carriage in the desired manner is then guided in the slot. In the region of the center of the slot, as a precaution, a separating web may also be arranged between the lower and the upper inside edge of the slot in order to avoid a mutual contact of the guiding carriage of the two gripper rails moving in opposite directions.
Alternatively, it is provided that the driving and guiding wheels of the chain have vertically extending axles and that the guide rails extend in parallel next to one another. This design has the advantage that biased pivoted levers are not required. Advantageously, one of the guide rails in the area of the center of the side edge of the printing table, is placed higher than the adjacent guide rail by a degree that corresponds to at least approximately the height of one gripper rail. In this manner, when the gripper rails pass through the path of the guide rails which are arranged in an oval, the gripper rails run above one another in the center of their paths in the desired manner of motion.
In order to achieve this, it may be provided that the pulleys of the guide parts, are connected with carrying parts for the ends of the gripper rails via vertically extending support parts. The support parts are guided vertically and slidably in driving means which are firmly connected with the chain. The carrying parts are connected with the support parts via shifting elements permitting a horizontal motion of the support parts by the degree of the distance of the adjacent ends of the chain. In a simple manner, the carrying parts may be designed as link tongues, and the shifting elements may be designed as swivel arms that can be rotated around a vertical axis. The swivel arms are connected with the assigned support part, such that the swivel point of the swivel arms at the link tongues will then be located in a vertical plane going through the axles of the deflecting and driving wheels. A simple realization of this embodiment can be achieved when the support parts are designed as cylindrical support rods which are guided rotatably and slidably in bushings serving as driving means and the swivel arms are firmly connected with the support rods. In order to achieve a better guiding of the support rods via a longer bushing, it is also contemplated that the bushings be fastened at two jointly driven chains disposed below one another in horizonatal planes, thus having a greater length.
Further objects, features, and advantages of the present invention will be come more obvious from the following description when taken in conjunction with the accompanying drawings.